The present invention relates to ferromagnetic self-regulating heaters. More particularly, the present invention relates to ferromagnetic self-regulating heaters with secondary performance enhancing layers.
This application relates to autoregulating ferromagnetic heaters of the type described in U.S. (Pat. No. 4,256,945 to Carter and Krumme; the parts of the disclosure relating to skin effect, skin depth and autoregulating ratios being incorporated herein by reference.
Autoregulating heaters using a high resistivity, high permeability magnetic surface layer on a non-magnetic magnetic low resistivity substrate have been developed to a point where they are useful in a variety of applications. Their successful operation depends on their ability to contain the current in the magnetic surface layer which is also the heating layer since it has a high resistivity. Thus a magnetic surface layer having both high permeability and high resistivity is required. It must also have the proper Curie temperature for the intended application. One disadvantage of this scheme is that at high power levels the magnetic fields in the surface magnetic layer may be very high, in some applications of the order of many oersteds, causing the effective permeability to be relatively low due to saturation.
Also, the power factor (PF) of the impedance of the magnetic surface layer heaters described above is relatively low e.g., 0.7 at temperatures below Curie, leading to the necessity of using reactive power factor correction elements in the tuning circuit. The power factor behavior of a design shows the approach of the power factor to a maximum value of 0.707 as the magnetic layer thickness increases.
The present invention provides a means for overcoming the above restrictions by adding further layers of material. Many improvements occur from this additional layer; high power factor below Curie, simplifying impedance matching; more flexibility in the overall design, including the requirements on the magnetic layer; higher effective permeability in the magnetic layer; a broad frequency range over which good performance, i.e., high self-regulation (S/R) ratio and high power factor are maintained.
The self-regulation (S/R) ratio is an important parameter in autoregulating heater design. This ratio refers to the ratio of overall resistance of the heater below effective Curie to the heater resistance above effective Curie. This change in resistance coupled with a constant current causes the heater to generate drastically less heat for a given amount of current when the temperature of the heater is above Curie. Therefore, the magnitude of the S/R ratio determines the effectiveness of autoregulation.
Jackson and Russel in U.S. Pat. No. 2,181,274 use a sheath of non-magnetic material (they suggest brass) on a magnetic material base. They couple to this structure inductively. Conditions for maximum efficiency, or maximum power factor, or the best possible combination of efficiency and power factor are disclosed. Jackson does not claim an ohmicly connected heater nor mention self-regulation. Jackson's approach which uses low frequencies does not mention or use Curie temperature self-regulation and does not appear to take advantage of the improved effective permeability of the ferromagnetic material; a factor of great importance in effective autoregulation.